Apparatus and methods for heat treating materials and incinerating vaporous off-products

ABSTRACT

Material to be heat treated is passed through a rotary-tray dryer held at high temperature by burning a combustible fuel in a burner system. The vaporous off-products generated by treating of the material are withdrawn from the dryer, intermixed with outside combustion air and conveyed through a condenser, where condensable constituents are in large part removed, to the burner system to effect combustion of the fuel and incineration of noncondensable, noisome constituents of the vaporous offproducts. The volumetric flow rate of air-vapor mixture delivered to the burners is maintained at a substantially constant value by a damper adjusted automatically in response to fluctuation of the mixture pressure at the burners from a predetermined level. A generally T-shaped duct connects the condenser to the dryer and to the atmosphere, and a blower interposed between the condenser and the burners creates a negative pressure on the upstream side productive of an inflow of combustion air from the atmosphere and an outflow of vaporous off-products from the dryer. Adjustable dampers are provided in the T-shaped duct to regulate the pressure drop thereacross so as to establish desired pressure levels within the dryer and at the condenser inlet. Temperature control within the dryer is effected by regulating the flow of fuel to the burners in accordance with variations in dryer temperature. A provision for over-temperature control shuts off the main fuel supply to the burners in the event the dryer temperature becomes excessively high. A flame safety system shuts down the apparatus if certain crucial conditions, such as pilot flame failure, occur.

United States Patent 1191 Worden, Sr. et al. 1 51 Apr. 24, 1973 APPARATUS AND METHODS FOR [57] ABSTRACT HEAT TREATING MATERIALS AND Material to be heat treated is passed through a rotary- INCKNERATING VAPOROUS tray dryer held at high temperature by burning a com- PRODUCTS bustible fuel in a burner system. The vaporous off- [75] Inventors: George M. Worden, Sr., Park Ridge; prfducts generated by treating the material a Alexander M. Lane Anendale, both withdrawn from the dryer, 1nterm1xed with outside OfNJ combust1on a1r and conveyed through a condenser,

where condensable constituents are, in large part [73] Assignee: Wyssmont Company, Inc., Fort Lee, removed, to the burner system to effect combustion of NJ. the fuel and incineration of non condensable, noisome v constituents of the vaporous off-products. The volu- [22] Filed 1971 metric flow rate of air-vapor mixture delivered to the [21] Appl. No.: 199,273 burners is maintained at a substantially constant value by a damper adjusted automatically in response to fluctuation of the mixture pressure at the burners from [52] US. Cl. ..34/32, 34/47, 34/48, a predetermined leveL A generally T shaped duct com 34/73 34/79 34/86 34/185 A nects the condenser to the dryer and to the at- [51] lltt. Cl ..F26b 3/00 mosPhere and a blower interposed between h [58] Field of Search ..34/32, 47, 48,73, denser and the burners Creates a negative pressure on 34/79, 86, 110/8 1 202 the upstream side productive of an inflow of combustion air from the atmosphere and an outflow of 1 References Cited vaporous off-products from the dryer. Adjustable dampers are provided in the T-shaped duct to regulate UNITE D STATES PATENTS the pressure drop thereacross so as to establish desired 2,744,477 5/1956 Hartley et al. ..110/8 A Pressure levels Within the dryer and at the Condenser 3,314,159 4/1967 Bell ..1 /8 A inlet. Temp rature control within the dryer is effected 3,601,400 8/1971 Erismin et al "110/8 A by regulating the flow of fuel to the burners in ac- Primary ExaminerWilliam FJODea Assistant Examiner-Peter D. Ferguson Attorney-Granville M. Brumbaugh et al.

cordance with variations in dryer temperature. A

provision for over-temperature control shuts off the main fuel supply to the burners in the event the dryer temperature becomes excessively high. A flame safety system shuts down the apparatus if certain crucial conditions, such as pilot flame failure, occur.

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a a 77% I54 72 44 J I92' I 11 5 a l N msmfil I J H i I64 1 I5 M I I l I [N GI- B ',A i ZDBI I Tl? I: l l.msmuum ;l72 1 1 1 t l| AIR I IbO "m f IIBb I66 1:176 I 1 l A[0 i gens U i 0 l4 V 1201: [22 i l v i I56 1 231 m I I30 n 1 DRYER} i g-g I36b 15: t 134 202 l-20a I32 To 14 OTHER C H BURNERS M H42 I8 0 5L. I38 uo- Patented April 24, 1973 2 Sheets-Sheet l mm av APPARATUS AND METHODS FOR HEAT TREATING MATERIALS AND INCINERATING VAPOROUS OFF-PRODUCTS BACKGROUND OF THE INVENTION This invention relates in general to the heat treatment of materials and, in particular, to improved apparatus and methods for heat treating materials and for incinerating noncondensable vaporous off-products released by the material during treatment to eliminate odors or other offensive or noxious properties of such off-products.

In manufacturing or processing procedures involving the heat treatment of materials for drying or other purposes, gaseous or vaporous off-products are frequently produced which have extremely unpleasant characteristics, such as, for example, strong odors, noxious or irritating constituents, and the like. When discharged to the atmosphere, such off-products often have a serious nuisance effect in the surrounding community as well as in the facility generating the off-products. In addition, dispensing these products directly to the atmosphere in many instances poses an air pollution threat of considerable consequence It is important, therefore, that equipment and techniques be developed for eliminating the foregoing undesirable aspects associated with such heat treatment processes in a practical and economical manner. This need is especially pressing in light of the trend in recent years toward the imposition by federal, state and local governments of more and more restrictive pollution control standards.

The present invention fulfills these and other requirements of the prior art.

SUMMARY OF THE INVENTION In accordance with the invention, material is treated at high temperature upon being passed through a dryer that is sealed against substantial inflow of atmospheric air. Heatis supplied to the dryer interior by burning a combustible fuel in a burner system associated with the dryer. The vaporous off-products, including malodorous or otherwise noisome constituents, are withdrawn from the dryer, intermixed with outside combustion air, and conveyed to a condenser, scrubber or other suitable device for removal of condensable elements. The condenser outlet connects through a blower to the burner system, so that the noncondensable off-products-combustion air mixture exiting from the condenser is forced to the burners to effect combustion of the fuel and incineration of the noncondensables, thereby meeting the dryer heat load and eliminating odors and other offensive properties of the off-products. In this way, only a relatively small volumetric air flow is delivered to the burner system, and fuel-air ratios required for firing the dryer to proper temperatures for treatment and for incinerating noncondensed off-products may be achieved with low fuel consumption and reduced power demand by accessory equipment.

According to one feature of the invention, the volumetric flow rate of the air-vapor mixture to the burners is maintained at a substantially constant value by flow controls responsive to fluctuation of the mixture pressure at the burners. Heat release to the dryer is then governed by varying the rate of supply of fuel in accordance with changes in dryer temperature. This allows stabilization of the pressure within the dryer at an optimum level simply by adjusting the cross sectional flow area of the flow paths leading from the dryer to the condenser on the one hand and between the atmosphere and the condenser on the other. Preferably these adjustments provide for slightly negative pressures within the dryer and at the inlet to the condenser, thus preventing odoriferous gases from escaping through the combustion air intake or through points of seepage in the dryer housing.

Venting of untreated noisome products through the burner stacks is also substantially precluded by selecting and controlling the flow rates of fuel and combustion air to the burners such that the quantity of excess air and the flame temperature are always sufficient to incinerate all of the combustible noncondensed vapors reaching the burners. Since the combustion air demand of the burners is relatively small, this can be accomplished with low fuel and power consumption.

In addition to the temperature responsive controls for regulating combustion fuel supply, a second temperature-actuated system is provided to shut off combustion fuel flow when the dryer temperature reaches a preset level considered too high for continued operation without damage to dryer components due to overheating.

Also provided, as another feature of the invention, is a flame safety system for sensing, among other parameters, the presence of a pilot flame in each burner of the burner system and, absent any one of such flames, for shutting down the system until normal operating conditions are restored.

In a preferred construction of the dryer, a plurality of vertically superimposed rotatable trays carry the material to be treated from an inlet in an upper region of the dryer to an outlet in a lower region. Wipers fixed to the dryer housing wipe the material from the trays upon each revolution thereof, so that the material advances successively from tray to tray through the dryer from the inlet to the outlet. Preferably the duct conveying the vaporous off-products from the dryer to the condenser has a generally T-shape, its stem connecting to the condenser, one of its arms connecting to a vapor discharge opening in the upper dryer region and the other arm connecting to the atmosphere. A damper located in each arm of the T allows the aforementioned adjustment of the cross section flow areas for the vaporous off-products and combustion air.

By virtue of the foregoing construction and operation, heat treatment of a wide variety of materials may be accomplished efficiently and economically and without dumping noisome exhaust gases to the atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be made to the following description of an exemplary embodiment, taken in conjunction with the figures of the accompanying drawings, in which:

FIG. 1 is an elevational view of a drying and incinerating apparatus constructed according to the invention, with parts broken away for clarity of illustration;

, FIG. 2 is a plan view of the dryer and incinerator apparatus of FIG. 1;

FIG. 3 is an enlarged vertical sectional view taken along the line 33 of FIG. 2, looking in the direction of the arrows:

DESCRIPTION OF AN EXEMPLARY EMBODIMENT Although a specific embodiment of the invention is described herein with reference to the drying of wet materials, this is done solely by way of illustration of one suitable application of the invention. Generally, the invention may be applied to any process involving the heat treatment of materials in the course of which undesired vaporous off-products are produced. For example, it may be adapted for use with processes involving the purification of solids by sublimation, solvent recovery, the reaction of gases with solids, and the like.

In a similar vein, while the volatiles released as the material dries are referred to as vaporous off-products, it will be understood that no limitation in the nature of the volatiles, such as to water vapor, steam etc., is intended, and that all gases, fumes, vapors, and the like, of whatever origin within the material are encompassed by that term.

In fundamental form, a drying and incinerating apparatus constructed in accordance with the invention includes (see FIGS. 1 and 2) a dryer 10, a burner system 12 for supplying drying heat to the dryer, a condenser 14 for removing condensable constituents from the dryer effluent, a generally T-shaped duct 16 connecting the condenser to the dryer and to the atmosphere, and a blower 18 for taking in outside combustion air, evacuating the dryer of vaporous offproducts and forcing the vaporous off-products and combustion air to the burner system.

The dryer preferably is of the type manufactured under the trademark TURBO-Dryer by the Wyssmont Company, Inc., 1470 Bergen Boulevard, Fort Lee, N..I., the assignee of the present application. The TURBO- Dryer is a continuous tray dryer having a plurality of vertically superimposed, annular trays 20 supported for rotation within a housing 21 by a cage 22 fixed to a two-part shaft 24. Wet material to be dried is introduced into the dryer through an inlet chute 26, dropping onto the upper trays 20 as they rotate beneath the inlet. As is more fully described hereinafter, the rate of material input is carefully controlled to assure proper loading of the dryer. A material feeding device (not shown), preferably sealed against the escape of vaporous off-products back through the device, is provided in association with the inlet 26 to deliver the material to the dryer in the proper quantity. For certain materials, it may be desirable to use a pre-breaker screen and grinder in advance of the feeder to condition the material. If so, these components preferably are vented to the housing interior to capture any malodorous gases or dust resulting from the material conditioning steps.

Upon each revolution of the trays 20, the material is advanced downward from tray to tray by the action of a stationary arm 28 which wipes the material carried by each tray through radial slots (not shown) in the trays. The material falling to the next lower tray is spread evenly over the tray surface by a stationary leveler arm 30 to expose a greater surface area of the material. It will be appreciated that a wiper arm 28 and a leveler arm 30 are positioned in overlying relation to each tray 20, except the uppermost tray, although for purposes of clarity only one arm of each type is depicted in FIG. 1.

The material is thus advancedprogressively through the dryer by repeated wiping and leveling and is discharged through an outlet chute 32 (see FIG. 4) in the housing bottom to a carry-off conveyor 34. This conveyor is also sealed against the escape of vaporous off-products. A sweeper arm 36 attached to the lowermost tray sweeps all of the material on the dryer bottom into the outlet 32.

Vapor emanating from the material is circulated horizontally between the trays 20 by turbofans 38 carried by shaft 40. In a preferred arrangement, the trays 20 and turbofans 38 are rotated at different speeds; accordingly, separate shafts, bearing supports and drive trains are provided for each. Thus, the cage 22 supporting the trays 20 is fixed at its lower end to an annular turntable 42 (see FIG. 4) that rests on a circumferential flange 44 formed on the lower part of the shaft 24. A thrust bearing.46 rotatably supports the cage and tray assembly on underlying structural framing 48. Rotation of the trays 20, then, is by means ofa ring gear 50, that is splined or otherwise fixed to the shaft 24 and drivably connected through a pinion gear 52, a variable speed worm reducer 54 and a chain sprocket 56 to an electric motor (not shown) or similar prime mover.

As shown in FIG. 4, the fan shaft 40 is journalled within the lower part of the two-part shaft 24, and extends through a thrust bearing 58. A coupling 60 connects the shaft 40 to a variable speed worm reducer 62 that, like the worm reducer 54, is driven by a sprocket 64 and chain connection with a prime mover (not shown).-

The rotational speeds of the trays 20 and the turbofans 38 may therefore be independently adjusted to afford optimum drying conditions in respect of retention time of the material on the trays and the velocity of the vaporous off-products over the surface of the material. Similarly, the setting of the leveler arms 30 may be varied to provide material layers of various thickness on the trays.

A pressurized filtered air purge of the thrust bearings 46 and 58 prevents contamination of the bearings by product dust or fines. Additionally, the purge air assists in cooling of the bearings and, by virtue of its pressurized state and flow into the interior of the dryer, prevents the hot vapors inside the dryer from condensing in the bearing. In addition, an air purge, also portrayed schematically in FIG. 4, of the area 66 beneath the turntable 42 may be provided to prevent weeping of vapors into that area, where they might contrayed in FIGS. 1 and 4 as extending over nearly the full height of the dryer, they may be terminated at a higher location to allow for cooling of the material in the lower trays.

A combustible fuel, such as natural gas or oil, is supplied to the burners 70 through a main fuel manifold (not shown in FIGS. 1 and 2) where it mixes with combustion air flowing from a main air manifold 72, and is ignited. The combustion products produced in the burners 70 flow through the heaters 68 and are exhausted to the atmosphere through individual stacks 74 or through an exhaust manifold connecting with a common stack. Although not shown in FIGS. 1 and 2, a pilot fuel manifold and a pilot air manifold are also coupled to each burner assembly 70 to maintain a pilot flame in the burners independently of whether fuel or air is being supplied to the burners through the main fuel manifold or main air manifold. A more complete description of the fuel and air supply stream is presented hereinafter in connection with FIGS. 5.

The burners 70 are intended to be operated at a constant volumetric flow rate of combustion air, with control of the drying temperature being effected through regulation of the fuel flow rate. The control system provided for this purpose is described in detail hereinafter.

As the wet material progresses through the dryer and is exposed to the high dryer temperatures, water and other liquids in the material are vaporized, filling the dryer housing with vaporous off-products. Typically most of the vapor load of the dryer would be water vapor in the form of saturated or superheated steam. Other vapors may be present in greater amounts, of course, depending upon the liquids with which the material is wetted. Normally, the noncondensables requiring incineration constitute only a small part of the total vapor load.

In any event, the vapors form a high temperature drying atmosphere within the housing 21 that, upon being circulated over the trays by the turbofans 38, contributes greatly to rapid drying of the material. It is a feature of the invention that substantially no outside air is admitted to the dryer housing for this purpose and, indeed, the housing 21 is sealed against such inflow of air aside from the previously mentioned purge air. A three-fold advantage is thereby realized, first, fuel consumption is minimized inasmuch as the outside air required for combustion is not heated to the drying temperature and subsequently cooled in the condenser, with resulting heat loss. Second, the condenser size and load is reduced in that cooling of outside air, preheated to drying temperatures, is not required. Third, duct sizes are smaller since outside air volume is not increased by preheating.

Vaporous off-products pass from the dryer through an exhaust port or opening near the top of the housing which communicates with one arm 76 of the generally T-shaped duct 16, the other arm 78 of which is open to the atmosphere.

The arms 76 and 78 of the T-shaped duct 16 are equipped with dampers 88 and 90, respectively, to allow control of the pressure drops thereacross. The dampers preferably are of the type having opposed vanes 92 (see FIG. 3), and are each manually adjustable, through an operator rod 94 and handle 96, to the desired setting. Since the volumetric flow rates through the respective duct arms 76 and 78 are held substantially constant, manipulation of the dampers 88 and 90 permits ready adjustment of the pressures within the dryer housing 21 and T-shaped duct 16 to optimum levels, as is more fully described below.

The stem 98 of the T-shaped duct 16 connects to the inlet end of the condenser 14, conveniently of the conventional shell and tube type, for removing condensable vapors from the air-vapor mixture formed in the duct 16. Cooling water is circulated through the condenser 14 by means of appropriate piping connections 100 and 102. A spray nozzle (not shown) may be positioned within the T-shaped duct 16 to flush the tube sheet of the condenser 14 to prevent its being clogged by particulates entrained in the air-vapor mixture. Appropriate connections are, of course, provided to supply water in sufficient quantity to the spray nozzle.

Vapors condensed in the condenser 14, together with the water injected by the spray nozzle, are collected in a condensate tank 104 and are drawn off through a drain 106 to waste or to recovery or recycling equipment, depending on the value of the condensate. A connection 108 is provided near the top of the condensate tank 104 for liquid level control or for the provision of a spray nozzle for cleaning solids from the tank. One or more additional connections might be provided for these purposes.

Noncondensable constituents of the air-vapor mixture flow through the tank 104 and are passed through a mist eliminator i.e., a mechanical type filter (not shown), housed in a right-angle duct 110 joined through a reducer section 112 to the input side of the blower 18. The blower functions as the main combustion air blower and may be of any type suitable for use with combustion systems. The upstream mist eliminator also serves to protect the blower, as well as the burner system 12, from contamination by air born particulates.

On its output side, the blower 18 connects to an accordion type expansion coupling 1 14 leading to a vertical pipe 116 that conveys the combustion air and noncondensable off-products, now under pressure from the blower, to the main air manifold 72 for the burner system 12. Odors or other offensive characteristics of the vaporous off-products are in this way eliminated, so that the combustion products discharged through the stacks 74 are free of stench, pollutants or other noisome properties. I

As another feature of the invention, the foregoing drying and incineratirig processes are carried out on an essentially automatic basis and within predetermined optimum parameters. .Appropriate operating control systems are provided for this purpose, as are separate safety systems operative in response to certain potentially harmful or dangerous contingencies for rapidly curtailing operation so asto restore safe operating conditions. The functional arrangement of the several operating and safety systems is illustrated diagrammatically in FIG. 5. There, a drying and incinerating apparatus having a burner system incorporating a total of IS burners 70 is depicted. The fuel is assumed to be natural gas.

. Turning first to the burner system 12, the main burner assembly 70a is shown connected through a combustion gas feeder line-118a to-the main gas line 1 19 through a pilot gas feeder line 120a to the pilot gas line 122. The pilot gas feeder line 1200 leads to a pilot unit 124a in the burner assembly 70a. Similar combustion gas feeder lines 1 18b and pilot gas feeder lines l20b are provided for each of the other 14 burners, as is indicated in FIG. 5. The main gas line 119 and the pilot gas line 122 connect through a plug-cock 126 to a gas supply source.

As previously mentioned, the combustion air for the burner system is taken in through the duct arm 78 (see the arrow in FIG. of the T-shaped duct 16, intermixed with the vaporous off-products flowing from the dryer and thereafter passed sequentially through the condenser 14, tank 104, duct 110, blower l8 and pipe 116 to the main combustion air manifold 72, from where it is delivered to the main burner assembly 700 through a connecting air line 128a and to the other burners through similar connecting air lines 128b, etc.

Air for the pilot unit 124a is supplied through a separate inlet, a filter 130, a blower 132, a pilot air line 134 and a connecting pilot air feeder line 136a. Additional pilot air feeder lines 136b, etc., lead from the line 134 to the pilot units of the remaining burners.

The combustion air flow to the burner system 12 is kept at a constant value by maintaining a constant airvapor mixture pressure at theburner assemblies 70. To this end, a pressure-indicator-controller (PIC) is connected to a pressure mp 137 in the main burner assembly 70a. The output of the PIC actuates an operator 138 for an opposed-vane damper 140 interposed in the duct 110 upstream of the main air blower 18. The operator 138 may be of the pneumatic variety, such as a reversible air motor, and may be coupled to the PIC through a sealed air circuit 142, including an instrument air supply set 144.

In operation, the PIC responds to fluctuation of the mixture pressure at the main burner assembly 70a from a preset level to cause the pneumatic operator 138 to open or close, as the case may be, the vanes of the damper 140 so as to increase or decrease the cross sectional flow area of the damper 140 to the extent needed to maintain the combustion air flow rate substantially at the predetermined value. Combustion air will therefore be delivered to the burner system at a substantially uniform rate of flow at all times.

Since the- PIC monitors the combustion air-vapor mixture pressure only at the main burner assembly 70a, pressure matching of the other 14 burner assemblies is required, this being accomplished by manual setting of a butterfly valve 146 included in each burner assembly. A pressure gauge 148 is provided at each assembly for this purpose. A typical design pressure of the combustion air-vapor mixture at the burner head assemblies is on the order of ten inches of water, gauge. A differential manometer 150, orifice 151 and a dial thermometer 152 may be provided at convenient locations in the pipe 116 and the duct 110, respectively to afford additional information as to the velocity and temperature of the air-vapor mixture flow upstream of the burner system and to allow determination of the total volumetric-flow rate of the mixture.

Regulation of the main gas flow to the burner system is in accordance with the drying temperature desired within the dryer 10, and specifically at the upper region of the dryer. However, it is desirable as well to have a an adjustable port valve 168 located in the main gas line 119. The TRC preferably comprises a two-pen pneumatic recorder-controller, with control on one pen over a given temperature range and an adjustable proportional band.

The upper temperature sensor 154 is connected to the controlling pen of the TRC, while the non-controlling pen is hooked to the lower sensor 156. The

lower limit (high fire setting) of the controlling pen may then be fixed at a temperature, e.g., 350 F., below which the drying temperature should not fall. The closer the dryer temperature approaches the lower limit, the farther the port valve 168 will be opened by the actuator 166 in response to input signals from the TRC, thus supplying a greater quantity of gas to the burner system and hence more heat to the dryer. Conversely, as the drying temperature approaches the upper limit (low fire setting), e.g., 410 F., set for the controlling pen, the valve 168 will be closed to curtail gas flow to the burners and thus to reduce the heat input to the dryer. Proportional firing is afforded by the TRC at'temperatures between the upper (low fire) and lower (high fire) limits. By this control arrangement, the dryer temperature tends always to be maintained within a preset, optimum range.

As already noted, it is desirable to have a minimum adiabatic flame temperature in the burners sufficient to incinerate all of the noncondensable off-products. This minimum temperature is approximately 1,500 F. Consequently, the upper limit of the TRC controlling pen should be selected to provide a gas flow rate through the main gas line 1 19 such that the excess air at the burners is preferably no greater than 200 percent, i.e., excess air in amounts greater than 200 percent willresult in adiabatic flame temperatures less than the 1,500 F. minimum required for incineration of the noncondensable off-products. The lower limit approaches 0 percent excess air, i.e., there must be sufficient oxygen for complete combustion of the fuel and the noncondensables. The two limits of air fuel ratio are dictated by sufficient temperature for incineration and sufficient oxygen for complete combustion.

With constant air-vapor mixture flow, the fuel flow is variable over a range of 3 to 1, corresponding to 0 percent excess air and 200 percent excess air, respectively. Therefore, the total heat output of the burner system varies over the range of 3 to 1, corresponding to the fuel flow rate. Consequently, the dryer system will hold includes radiation losses, heat of vaporization, sensible heat, heats of formation on reaction, etc. Further, one skilled in the art can determine the minimum dryer heat load which will cause the dryer temperature to rise excessively, thereby activating the over-temperature control system described hereinafter. Similarly, the maximum dryer heat load, above which the dryer tem' perature would fall too low for proper drying, can be determined by one skilled in the art.

It is further noted that one skilled in the art will include the heating value of the offproducts to be incinerated in setting the air-vapor mixture flow, as well as providing sufficient oxygen for such incineration over and above that required for complete combustion of the fuel.

Having determined the required air-vapor mixture flow rate, the set point of the PIC is adjusted to the pressure at the main burner assembly 70a found to cor respond to that flow rate. Pressure matching at the other burners is then accomplished through use of the valves 146 and gauges 148, as noted. Hence deviation of the mixture pressure at the main burner assembly 70a will be detected by the PIC and transmitted to the damper operator 138, with the result that the damper 140 will be opened or closed to maintain a substantially constant air-vapor flow rate to the burner system.

High fire gas flow to the burners is provided by manually adjusting the setting of the adjustable port valve 168 while holding the automatic setting of the valve in the high fire position, i.e., the position corresponding to the lower limit of the controlling pen of the TRC. Should the dryer temperature at the upper sensor 154 drop below the lower limit, the TRC will thus actuate the pneumatic operator 166 to open the port valve 168 to the maximum, or high fire, position. The gas pressure at each burner should also be matched with that at the main burner assembly 70a. lndividual valves 170 at each burner allows this to be done. The pressure gauges 147 may be temporarily used for this purpose.

The high fire gas and air flow rates should be such that the heaters 68 will always deliver a heat release to the dryer greater than the dryer load at high fire conditions, or, in other words, at the most unfavorable drying conditions. This ensures that the material will be dried to the design criteria by the time it exits from the dryer.

Lower input rates of material or a lower initial moisture content will produce lower dryer loads, and thus will require less fuel and air input. At a minimum, however, the material feed rate and moisture content should be high enough to insure that in incinerating the gaseous off-products, the dryer is not overheated.

The minimum feed rate may be derived by estimating the minimum acceptable combustion air flow through the intake opening 80 in the T-duct arm 78. This may be determined by observing the intake opening 80 and adjusting air-vapor mixture flow to the burners downward until wisps of vapor emanate from the opening 80. The mixture flow is then increased until vapor no longer seeps from the opening. This is the minimum combustion air flow rate; otherwise, some odoriferous or noxious off-products may backflow out of the air intake and escape untreated. Moreover, the pressure within the dryer is controlled by the combustion air flow through the T-shaped duct 16 and may become positive if too little air is taken in.

To avoid damage tothe dryer or other components of the apparatus due to overheating, a temperature-indicator limiter (TIL) is provided to monitor the dryer temperature, as by a sensor 172 and capillary line 174, and in response to an increase of the temperature above a preselected value, e.g., 450 F., to actuate a motorized valve 176 in the main gas line 1 19 to shut off the main (combustion) gas flow to the burners. Inasmuch as the pilot gas line 122 remains open, notwithstanding that the over-temperature valve 176 is closed, the flow of pilot gas to the burner system is not interrupted. The burners will therefore reignite automatically when the dryer temperature falls to a safe level. In event of a shut down of the combustion gas flow due to excessive dryer temperature, the main combustion air flow is continued and consequently some vaporous off-products could be vented to the atmosphere. However, the concentration of such untreated vapors escaping will be minimal inasmuch as the TIL can be activated only when the feed rate of wet material to the dryer is well below design capacity.

Onoff control of all gas to the apparatus may be had through manipulation of the plus cock 126 upstream of both the main gas line 119 and the pilot gas line 122. 1 Because of the rate of intake of outside air through the opening is substantially constant, the pressure within the dryer 10 may be stabilized quite readily simply through adjustment of the T-duct dampers 88 and (see FIG. 5). With the dryer on line and operating at full design capacity, the combustion air damper 90 is set to provide a negative pressure at the condenser. The vapor damper 88 is then adjusted to establish a pressure near the top of the dryer housing that is higher than that at the condenser but less than atmospheric. In practice, the pressure at the top of the dryer should be as close to atmospheric as possible while still being measurably negative, e.g., minus 0.01 inches of water, gauge. This provides for proper flow of the vaporous off-products to the T-duct 16 without imposing unduly high pressure loads on the dryer-walls. It also prevents the escape of noisome vapors at points of seepage around the dryer or through the air intake opening 80.

It will be understood that the foregoing assumes a vapor density within the dryer less than that of air.

Otherwise, a positive pressure might exist in the lower portion of the dryer even though the pressure in the upper portion is below atmospheric. Hence in heat treating processes which generate heavier-than-air vapors, the pressure at the lower dryer portion should be maintained measurably negative, but as close to atmospheric as possible. This will ensure that a negative pressure will exist throughout the dryer.

As mentioned, it is preferred that the pressure at the top of the dryer be held negative and as close to atmospheric as possible. To that end, the opening 80 (see FIG. 3) to the atmosphere in the arm 78 is surrounded by an inwardly converging flange 82 and is divided into quadrants by intersecting baffle-like dividers 84 extending between opposed walls of the flange 82. A screen 86 positioned across the opening 80 filters undesired solid materials, birds or the like from the incoming air. By virtue of this construction, the effect of wind conditions on dryer pressure is largely neutralized. Similarly, the likelihood of the opening 80 being blocked by debris, birds or the like is substantially reduced.

A flame safety system is also provided for effecting the lightoff of the burners, in unison, and for supervising the continued running of the burner system.

An schematically depicted in FIG. 5, such a system might include a flame safety panel (FSP) that is wired separately to each of the fifteen burners, with conductors 180a and 182a connecting the panel to the main burner assembly 70a and other similar conductors 180b and l82b leading to the other burners.

Lightoff of each pilot flame is preferably accomplished by direct spark ignition, and for this purpose an ignition transformer 184a is provided in the conductor 1820 to deliver the required electrical potential to the ignition plug (not shown) in the burner assembly. Flame safety is assured by using a flame rod 186 to sense the presence of the pilot flame and to relay this information to the FSP. When all pilot flames are proven, normal operation of the burner system may go forward. Flame failure at any one burner, however, results in a complete shutdown of the burner system. To that end, a main pilot gas valve 188 and a main gas valve 190, both preferably of the motorized type, are operatively connected to the FSP, as by conductors 192 and 194, respectively, and are closed down upon pilot flame failure at any one burner so as to shut off the flow of all fuel to the burner system. Should the main gas valve 190 be closed in this way, a three-way solenoid shut-off valve 196 in the air circuit 162 is also closed to stop the flow of operating air to the pneumatic operator 166 for the main adjustable port valve 168 so that light off will occur at the low fire setting. Upon reestablishing safe pilot flame conditions, the valves 188, 190 and 196 are returned to the normally open position.

Other parameters may be monitored by the flame safety system to further insure proper operation of the apparatus. For example, pressure limiting switches 198 and 200 (of the automatic reset type) may be interposed in the main gas line 119 downstream of and upstream of, respectively, the main gas valve 190, while another pressure switch 202 may be provided in the main air pipe 116 and still another 204 in the pilot air line 134. Each of the pressure switches 198, 200, 202 and 204 is wired in series with the others and with the FSP, as indicated by the conductor 206 in FIG. 5, so that all must be closed before full start up of the burner system may be accomplished. Adequate fuel and air for safe burner performance will thus always be delivered. ln actuality, switches 200, 202 and 204 are low pressure switches and operate to prevent initiation of the burner ignition sequence, or thereafter call for a stop to the burner system if fuel or air pressures fall below safe levels. Switch 198, on the other hand, responds to too high gas pressures to shut down the system. Should any one of the switches 198,200, 202 or 204 open, the FSP will close the main pilot gas valve 188 and the main gas valve 190.

Once the pilot flames and gas and air pressures are established, the FSP opens the motorized main gas valve 190, but slowly so as to increase combustion gas flow to the burners gradually. The TRC, however, sensing that the dryer temperature is below the lower limit setting, would want to call for high fire gas flow and, without more, would order the operator 166 to open the adjustable port valve 168 fully. lf permitted, such immediate full scale opening of the valve 168 could result in its being damaged. Accordingly, the three-way valve 196 in the air circuit 162 for the operator 166 is designed to override the TRC and limit the OPERATION The operation of the drying and incinerating apparatus described above will be evident to those skilled in the art from the foregoing description of the apparatus, but a further specific explanation of its preferred manner of operation is set out below for clarity in an understanding of the invention.

Prior to initiating operation, the PIC, TRC and TIL are adjusted in the manner previously described to provide the desired operating conditions in respect of fuel and combustion air flow rates, temperatures, and the like. The instrument air supplies 144 and 164, as well as the other accessory components of the control system, are of course put in readiness for use. The flame safety system is energized and all valves and switches placed in operating position.

Preliminary to starting the burners, purge air is supplied to the bottom bearings 46 and 58 and to the turntable 42. The purge air flows are maintained at all times. The tray drive system is then energized to begin rotation of the trays 20, and adjusted to fix the rotational speed at the desired operating r.p.m. As the next step, the turbofans 38 are started, with appropriate adjustment to the recommended operating speed.

The main combustion air blower 18 is then brought on line, circulation of the cooling water to the condenser 14 is begun and a flow of flushing water is supplied to the spray nozzle in the T-shaped duct 16. Start up of the pilot air blower 132 is also initiated.

The start button of the FSP is then depressed to open the main pilot gas valve 188 and commence flow of pilot gas to the pilot units 124 of the burners 68. Unison spark ignition of the pilot flames follows, and after each flame rod 186 proves its pilot flame, the main gas valve 190 is energized to allow combustion fuel flow to the burners. Assuming that all safety interlocks, such as the pressure switches 198, 200, 202 and 204 are properly closed, normal operation of the apparatus may go forward.

As the dryer temperature at the upper sensor 154 approaches the operating temperature, feeding of the material to the dryer is begun. Adjustment, if necessary, is then made to the damper in the air intake duct 78 to obtain the appropriate negative pressure at the inlet to the condenser 14. A similar adjustment is made to the damper 88 controlling the exhaust flow from the dryer 10 so as to provide a measurably negative pressure at the top of the dryer. Continuous operation of the drying and incinerating apparatus at the optimum rate is thereafter carried out on a substantially automatic basis by virtue of the functions of the previously described control systems, e.g., the PIC and TRC and their associated operators.

To shut down the apparatus, input of the material to the dryer is stopped but operation of the apparatus is otherwise continued to evacuate the dryer of vaporous off-products. The burner system 12 and the main combustion air blower 18 are then cut off, followed by deactivation of the remaining equipment except for the air purges to the bottom bearings 46 and 58 and the turntable 42. This shutdown procedure minimizes venting of the vaporous off-products to the atmosphere.

It will be understood that those skilled in the art that the above-described embodiment is intended to be merely exemplary in that it is susceptible of modification and variation without departing from the spirit and scope of the invention, as defined in the appended claims.

We claim:

1. Apparatus for heat treating materials and for incinerating noncondensable constituents of the resulting vaporous off-products comprising: I

a dryer having a housing substantially sealed against the inflow of atmospheric air;

an inlet in the housing for material to be treated;

an outlet in the housing for treated material;

a discharge opening in the housing for the vaporous off-products generated by heat treating of the material,

burner means defining at least one combustion chamber for burning a combustible fuel to supply heat energy to the interior of the dryer housing;

first duct means communicating with the discharge opening in the dryer housing and with the atmosphere for intermixing the vaporous offproducts with outside combustion air to produce an air-vapor mixture;

means for removing condensables from the air-vapor mixture and having an inlet and an outlet, the inlet being in communication with the first duct means downstream of the points of communication thereof with the dryer discharge opening and the atmosphere;

second duct means communicating with the outlet of the condensables removing means for conveying the air-vapor mixture discharged therefrom to the burner means to effect combustion of the fuel and incineration of the noncondensable constituents of the vaporous off-products; and

means responsive to fluctuation of the pressure of the air-vapor mixture at the burner means for maintaining the volumetric flow rate of air-vapor mixture delivered to the burner means at a substantially constant value.

2. Apparatus according to claim 1 further comprising blower means interposed in the second duct means for creating a negative pressure upstream thereof productive of an inflow to the first duct means of vaporous offproducts from the dryer and combustion air from the atmosphere.

3. Apparatus according to claim .1 further comprising means associated with the first duct means for regulating the pressure drop between the atmosphere and the inlet to the condensables removing means so as to provide a measurably negative pressure at the inlet to said removing means.

4. Apparatus according to claim 3 further comprising means associated with the first duct means for regulating the pressure drop between the interior of the dryer and the inlet to the condensables removing means so as 5 to provide a measurably negative pressure throughout the dryer interior.

5. Apparatus according to claim 4 wherein the means for regulating the pressure drop between the dryer interior and the inlet to the condensables removing means is adjusted such that the pressure at the uppermost portion of the dryer housing is higher than the pressure at the inlet to the condensables removing means but lower than atmospheric.

6. Apparatus according to claim 1 further comprising means responsive to fluctuation of the temperature within the dryer for controlling the volumetric flow rate of fuel supplied to the burner means so as to maintain the drying temperature within a predetermined range.

7. Apparatus according to claim 6 wherein the burner means is adapted to burn natural gas and the fuel supply control means is operable to vary the flow rate of gas supplied to the burner means within the range of from approximately 0 percent excess air to approximately 200 percent excess air.

8. Apparatus according to claim 1 further comprising means responsive to the temperature within the dryer reaching a predetermined level for shutting off combustion fuel flow to the burner means, thereby protecting the dryer against damage due to over-heating.

9. Apparatus according to claim 1 whereinthe means for maintaining the volumetric flow rate of the airvapor mixture at a substantially constant rate includes:

an adjustable damper interposed in the second duct means;

a motor operatively connected to the damper for adjustment thereof; and

actuator means for energizing the motor in response to fluctuation of the air-vapor mixture pressure at the burner means from a predetermined level.

10. Apparatus according to claim 1 further comprising:

independent means for supplying combustion fuel flow and pilot fuel flow to each combustion chamber of the burner means so as continuously to maintain at least a pilot flame therein; and

means responsive to the absence of the pilot flame in any combustion chamber for shutting off combustion fuel flow to the burner means.

1 1. Apparatus according to claim 1 wherein the first duct means comprises: 7

a generally T-shaped duct, the stem of which communicates with the inlet to the condensables removing means and one arm of which communicates with the discharge opening of the dryer housing and the other arm of which communicates with the atmosphere.

12. Apparatus according to claim 11 further comprising:

adjustable damper means located in the one arm for controlling the pressure drop thereacross so as to permit adjustment of the pressure within the dryer housing; and

adjustable damper means located in the other arm for controlling the pressure drop thereacross so as to permit adjustment of the pressure at the inlet to the condensables removing means.

13. Apparatus according to claim 12 wherein the damper means in the other arm is adjustable to maintain a measurably negative pressure at the inlet to the condensables removing means and the damper means in the one arm is adjustable to maintain the pressure within the uppermost portion of the dryer housing higher than the pressure at the inlet to the condensable removing means but lower than atmospheric.

14. Apparatus for heat treating materials and for incinerating noncondensable constituents of the resulting vaporous off-products comprising:

a dryer having a vertically elongated housing substantially sealed against the inflow of atmospheric air;

an inlet for material to be treated in an upper region of the housing;

an outlet for treated material in a lower region of the housing;

a plurality of vertically superimposed trays mounted for rotation within the housing;

means associated with the trays for progressively advancing the material from tray to tray from the inlet to the outlet;

a discharge opening in the dryer housing for the vaporous off-products generated by heat treating of the material;

burner means associated with the dryer defining at least one combustion chamber for burning a combustible fuel to supply heat energy to the housing interior;

first duct means communicating with the discharge opening in the housing and with the atmosphere for intermixing the vaporous off-products with outside combustion air to produce an air-vapor mixture;

a condenser for removing condensables from the airvapor mixture and having an inlet and an outlet, the inlet being in communication with the first duct means downstream of the points of communication thereof with the housing discharge opening and the atmosphere;

second duct means communicating with the condenser outlet for conveying the air-vapor mixture discharged from the condenser to the burner means to effect combustion of the fuel and incineration of noncondensables in the vaporous offproducts;

a blower located in the second duct means between the condenser and the burner means for creating a negative pressure upstream thereof productive of an inflow to the first duct means of vaporous offproducts from the dryer and combustion air from the atmosphere; and

means responsive to fluctuation of the air-vapor pressure at the burner means for maintaining the volumetric flow rate of air-vapor mixture delivered to the burner means at a substantially constant value.

15. Apparatus according to the claim 14 wherein the first duct means comprises a generally T-shaped duct, the stem of which communicates with the inlet to the condenser and one arm of which communicates with the discharge opening of the dryer housing and the other arm of which .communicates with the atmosph ere.

16. Apparatus according to claim 15 further comprising adjustable damper means located in the one arm for controlling the pressure drop thereacross so as to I permit adjustment of the pressure within the dryer housing; and

adjustable damper means located in the other arm for controlling the pressure drop thereacross so as to permit adjustment of the pressure at the inlet to the condenser.

17. Apparatus according to claim 14 wherein the means for maintaining the volumetric flow ate of the air-vapor mixture at a substantially constant rate includes:

an adjustable damper interposed in the second duct means;

a motor operatively connected to the damper for adjustment thereof; and

actuator means for energizing the motor in response to fluctuation of the air-vapor mixture pressure at the burner means from a predetermined level. 18. Apparatus according to claim 14 further comprising means responsive to fluctuation of the temperature within the dryer for controlling the volumetric flow rate of fuel supplied to the burner means so as to maintain the drying temperature within a predetermined range.

19.. Apparatus according to claim 14 further comprising means responsive to the temperature within the dryer reaching a predetermined level for shutting off combustion fuel flow to the burner means, thereby protecting the dryer against damage due to over-heating.

20. Apparatus according to claim 1 further comprising means associated with the first duct means for minimizing the effect on the pressure within the dryer of wind currents externally of the duct means.

21. Apparatus according to vclaim 14 further comprising means associated with the first duct means for minimizing the effect on the pressure within the dryer of wind currents externally of the duct means.

22. A method of heat treating material and incinerating noncondensable constituents of the resulting vaporous off-products comprising:

passing the material through a dryer substantially sealed against the inflow of atmospheric air;

burning a combustible fuel in a combustion chamber to supply heat energy to the interior of the dryer and thereby heat treat the material; conveying the vaporous off-products released by the material from the dryer along a first flow path; taking in outside combustion air through a second flow path and intermixing it with the vaporous offproducts to produce an air-vapor mixture; flowing the air-vapor mixture through a condenser to remove condensable constituents from the mixture;

delivering the air-vapor mixture discharged from the condenser to the combustion chamber to effect combustion of the fuel and incineration of noncondensable constituents of the vaporous offproducts; and

regulating the volumetric flow rate of air-vapor mixture delivered to the combustion chamber in accordance with the pressure of the air-vapor mixture at the combustion chamber so as to maintain the rate at a substantially constant value.

23. A method according to claim 22 further comprising regulating the volumetric flow rate of fuel supplied ing regulating the pressure drop across the second flow path to provide a negative pressure at the condenser inlet.

26. A method according to claim 25 further comprising regulating the pressure drop across the first flow path to provide a negative pressure within the dryer. 

1. Apparatus for heat treating materials and for incinerating noncondensable constituents of the resulting vaporous offproducts comprising: a dryer having a housing substantially sealed against the inflow of atmospheric air; aN inlet in the housing for material to be treated; an outlet in the housing for treated material; a discharge opening in the housing for the vaporous off-products generated by heat treating of the material, burner means defining at least one combustion chamber for burning a combustible fuel to supply heat energy to the interior of the dryer housing; first duct means communicating with the discharge opening in the dryer housing and with the atmosphere for intermixing the vaporous off-products with outside combustion air to produce an air-vapor mixture; means for removing condensables from the air-vapor mixture and having an inlet and an outlet, the inlet being in communication with the first duct means downstream of the points of communication thereof with the dryer discharge opening and the atmosphere; second duct means communicating with the outlet of the condensables removing means for conveying the air-vapor mixture discharged therefrom to the burner means to effect combustion of the fuel and incineration of the noncondensable constituents of the vaporous off-products; and means responsive to fluctuation of the pressure of the air-vapor mixture at the burner means for maintaining the volumetric flow rate of air-vapor mixture delivered to the burner means at a substantially constant value.
 2. Apparatus according to claim 1 further comprising blower means interposed in the second duct means for creating a negative pressure upstream thereof productive of an inflow to the first duct means of vaporous off-products from the dryer and combustion air from the atmosphere.
 3. Apparatus according to claim 1 further comprising means associated with the first duct means for regulating the pressure drop between the atmosphere and the inlet to the condensables removing means so as to provide a measurably negative pressure at the inlet to said removing means.
 4. Apparatus according to claim 3 further comprising means associated with the first duct means for regulating the pressure drop between the interior of the dryer and the inlet to the condensables removing means so as to provide a measurably negative pressure throughout the dryer interior.
 5. Apparatus according to claim 4 wherein the means for regulating the pressure drop between the dryer interior and the inlet to the condensables removing means is adjusted such that the pressure at the uppermost portion of the dryer housing is higher than the pressure at the inlet to the condensables removing means but lower than atmospheric.
 6. Apparatus according to claim 1 further comprising means responsive to fluctuation of the temperature within the dryer for controlling the volumetric flow rate of fuel supplied to the burner means so as to maintain the drying temperature within a predetermined range.
 7. Apparatus according to claim 6 wherein the burner means is adapted to burn natural gas and the fuel supply control means is operable to vary the flow rate of gas supplied to the burner means within the range of from approximately 0 percent excess air to approximately 200 percent excess air.
 8. Apparatus according to claim 1 further comprising means responsive to the temperature within the dryer reaching a predetermined level for shutting off combustion fuel flow to the burner means, thereby protecting the dryer against damage due to over-heating.
 9. Apparatus according to claim 1 wherein the means for maintaining the volumetric flow rate of the air-vapor mixture at a substantially constant rate includes: an adjustable damper interposed in the second duct means; a motor operatively connected to the damper for adjustment thereof; and actuator means for energizing the motor in response to fluctuation of the air-vapor mixture pressure at the burner means from a predetermined level.
 10. Apparatus according to claim 1 further comprising: independent means for supplying combustion fuel flow and pilot fuel flow to each combustion chamber of the burnEr means so as continuously to maintain at least a pilot flame therein; and means responsive to the absence of the pilot flame in any combustion chamber for shutting off combustion fuel flow to the burner means.
 11. Apparatus according to claim 1 wherein the first duct means comprises: a generally T-shaped duct, the stem of which communicates with the inlet to the condensables removing means and one arm of which communicates with the discharge opening of the dryer housing and the other arm of which communicates with the atmosphere.
 12. Apparatus according to claim 11 further comprising: adjustable damper means located in the one arm for controlling the pressure drop thereacross so as to permit adjustment of the pressure within the dryer housing; and adjustable damper means located in the other arm for controlling the pressure drop thereacross so as to permit adjustment of the pressure at the inlet to the condensables removing means.
 13. Apparatus according to claim 12 wherein the damper means in the other arm is adjustable to maintain a measurably negative pressure at the inlet to the condensables removing means and the damper means in the one arm is adjustable to maintain the pressure within the uppermost portion of the dryer housing higher than the pressure at the inlet to the condensables removing means but lower than atmospheric.
 14. Apparatus for heat treating materials and for incinerating noncondensable constituents of the resulting vaporous off-products comprising: a dryer having a vertically elongated housing substantially sealed against the inflow of atmospheric air; an inlet for material to be treated in an upper region of the housing; an outlet for treated material in a lower region of the housing; a plurality of vertically superimposed trays mounted for rotation within the housing; means associated with the trays for progressively advancing the material from tray to tray from the inlet to the outlet; a discharge opening in the dryer housing for the vaporous off-products generated by heat treating of the material; burner means associated with the dryer defining at least one combustion chamber for burning a combustible fuel to supply heat energy to the housing interior; first duct means communicating with the discharge opening in the housing and with the atmosphere for intermixing the vaporous off-products with outside combustion air to produce an air-vapor mixture; a condenser for removing condensables from the air-vapor mixture and having an inlet and an outlet, the inlet being in communication with the first duct means downstream of the points of communication thereof with the housing discharge opening and the atmosphere; second duct means communicating with the condenser outlet for conveying the air-vapor mixture discharged from the condenser to the burner means to effect combustion of the fuel and incineration of noncondensables in the vaporous off-products; a blower located in the second duct means between the condenser and the burner means for creating a negative pressure upstream thereof productive of an inflow to the first duct means of vaporous off-products from the dryer and combustion air from the atmosphere; and means responsive to fluctuation of the air-vapor pressure at the burner means for maintaining the volumetric flow rate of air-vapor mixture delivered to the burner means at a substantially constant value.
 15. Apparatus according to the claim 14 wherein the first duct means comprises a generally T-shaped duct, the stem of which communicates with the inlet to the condenser and one arm of which communicates with the discharge opening of the dryer housing and the other arm of which communicates with the atmosphere.
 16. Apparatus according to claim 15 further comprising: adjustable damper means located in the one arm for controlling the pressure drop thereacross so as to permit adjustment of the pressure within the dryer housing; and adjuStable damper means located in the other arm for controlling the pressure drop thereacross so as to permit adjustment of the pressure at the inlet to the condenser.
 17. Apparatus according to claim 14 wherein the means for maintaining the volumetric flow rate of the air-vapor mixture at a substantially constant rate includes: an adjustable damper interposed in the second duct means; a motor operatively connected to the damper for adjustment thereof; and actuator means for energizing the motor in response to fluctuation of the air-vapor mixture pressure at the burner means from a predetermined level.
 18. Apparatus according to claim 14 further comprising means responsive to fluctuation of the temperature within the dryer for controlling the volumetric flow rate of fuel supplied to the burner means so as to maintain the drying temperature within a predetermined range.
 19. Apparatus according to claim 14 further comprising means responsive to the temperature within the dryer reaching a predetermined level for shutting off combustion fuel flow to the burner means, thereby protecting the dryer against damage due to over-heating.
 20. Apparatus according to claim 1 further comprising means associated with the first duct means for minimizing the effect on the pressure within the dryer of wind currents externally of the duct means.
 21. Apparatus according to claim 14 further comprising means associated with the first duct means for minimizing the effect on the pressure within the dryer of wind currents externally of the duct means.
 22. A method of heat treating material and incinerating noncondensable constituents of the resulting vaporous off-products comprising: passing the material through a dryer substantially sealed against the inflow of atmospheric air; burning a combustible fuel in a combustion chamber to supply heat energy to the interior of the dryer and thereby heat treat the material; conveying the vaporous off-products released by the material from the dryer along a first flow path; taking in outside combustion air through a second flow path and intermixing it with the vaporous off-products to produce an air-vapor mixture; flowing the air-vapor mixture through a condenser to remove condensable constituents from the mixture; delivering the air-vapor mixture discharged from the condenser to the combustion chamber to effect combustion of the fuel and incineration of noncondensable constituents of the vaporous off-products; and regulating the volumetric flow rate of air-vapor mixture delivered to the combustion chamber in accordance with the pressure of the air-vapor mixture at the combustion chamber so as to maintain the rate at a substantially constant value.
 23. A method according to claim 22 further comprising regulating the volumetric flow rate of fuel supplied to the combustion chamber in accordance with the temperature within the dryer so as to maintain the drying temperature within a predetermined range.
 24. A method according to claim 22 further comprising shutting off combustion fuel flow to the combustion chamber in response to the temperature within the dryer reaching a predetermined level.
 25. A method according to claim 22 further comprising regulating the pressure drop across the second flow path to provide a negative pressure at the condenser inlet.
 26. A method according to claim 25 further comprising regulating the pressure drop across the first flow path to provide a negative pressure within the dryer. 